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Particle Size Distribution and Soil Texture It is important to know the texture of each soil horizon because this is used in several parts of the contest. Permeability: the most restrictive layer is what you consider for the permeability section. Usually, this is the bottom soil horizon. Thickness of the rooting zone: only a USDA texture of Clay will limit roots. No other textures are root limiting. This means that Sandy Clay is NOT root limiting. SHWT: sand and loamy sand are the only two textures in which you observe redox concentrations and stripping as indicators of SHWT. Regrouped texture classes for land judging The USDA textural classes have been regrouped for land judging. This means when you read something about loamy soils, there are 10 USDA classes this would be referring to. Let’s back up What is a soil particle? What is sand? What is silt? What is clay? You might think of sand as quartz and clay as kaolinite, and in fact, these are very common minerals, but the terms sand, silt, and clay specifically refer to particle size only. A small shell fragment is therefore sand if > 0.2 mm in size. That shell is mostly CaCO3, not quartz. A sand particle is simply one that is larger than 0.2 mm. Particles greater than 2 mm are considered rock fragments and not part of particle size analysis. Soil Particles Sand Silt . Clay Sand: 2-0.2 mm Silt: 0.2-0.06 mm Clay: <0.06 mm Particle Size Distribution (PSD) • The proportions of sand, silt, and clay in a soil. – E.g. 92% sand, 5% silt, 3% clay. • These proportions are based on the mineral portion only. • Organic matter not considered when determining particle size proportions • We determine PSD by allowing the particles to segregate themselves via settling in a water column. • We classify the texture using the determined proportions. Soil Texture • This is a classification system meant communicate the proportions of sand, silt, and clay. • Soils grouped in to classes with similarly behaving properties. • For example, the sand class is small because adding just a little bit of silt or clay changes the way the soil behaves. Soil Textural Classes Florida Textures: • Sand • Loamy Sand • Sandy Loam • Sandy Clay Loam • Clay we just don’t have much silt in FL Soil Textural Classes If you have lab data (e.g. the exact % Sand, silt, and clay, then use this triangle to determine the texture But you are in a contest, not a lab! • That’s right. You have to do this with your hands. • You are going to ball moist soil into your non-writing hand (don’t get your paper dirty!) • Use your thumb to work the soil into a ribbon. • The longer the ribbon, the further down the clay axis you are progressing. • When the ribbon breaks, you have your soil texture. Short ribbon Long ribbon Particle Size Estimated by Hand • • • • • Sandy particles feel gritty Silty particles feel silky smoth Clay partcles feel sticky You can shape and form soil into ribbons Then length of the ribbon is controlled by the sand content (more sand = shorter ribbon) • There is a flow-chart that can be followed to estimate the textural class of a soil sample Simple way of texture by feel Next slide is the correct way for all USDA textures Particle Size Estimated by Hand So you can see that the rapid, moderate, and slow permeability categories don’t currently line up directly with the broad textural groups of sanndy, loamy, and clayey. I’m actually looking to change this and have it line up. New manual in fall 2014.